Interpretive Summary: Grasslands across the United States play a key role in regional livelihood and national food security. Yet, it is still unclear how this important resource will respond to the regional drying and warming predicted with climate change. In this study, we investigated the impact of the early 21st century drought on six grasslands across the southwestern United States. We reported an exceptional decrease in grassland growth, and the replacement of native grasses with less-nutritious and more-fire-prone invasive grasses. By providing a model of grasslands’ response to prolonged warm drought and an operational means to monitor it, this analysis has important implications. First, ranchers have important new information for managing grasslands under predicted climate change to lower fire risk, minimize loss of forage, and retain ecosystem services. Second, the aberrant behavior reported here suggests that grasslands can serve as an early indicator of impending climate change. These compelling results in a natural setting at the regional scale should play a role in future grassland research, management and policy.

Technical Abstract:
Grasslands across the United States play a key role in regional livelihood and national food security. Yet, it is still unclear how this important resource will respond to the regional drying and warming predicted with climate change. The early 21st century drought in the southwestern U.S. resulted in hydroclimatic conditions that have been recognized as similar to those expected with climate change. In this study, we investigated the impact of this drought on above-ground net primary production (ANPP) of six Desert and Plains grasslands dominated by C4 (warm-season) grasses, and we used these results to generalize the functional response of C4 grasslands to predicted climate change. During prolonged warm drought, the ANPP decline in dry years was of a similar magnitude to the ANPP pulse in wet years, overriding the buffering mechanism reported in the late 20th century. In Desert grasslands, drought-induced grass mortality at the arid end of the moisture gradient led to shifts in the functional response to precipitation, and in some cases, new species assemblages occurred that included invasive grasses. In contrast, the Plains grasslands maintained significant linear relations between ANPP and PT throughout the prolonged warm drought. However, after 8 consecutive years of warm drought, the driest of the Plains grasslands showed indications of functional instability in this ANPP-PT relation. We conclude that Desert and Plains grasslands have different thresholds in functional response to altered hydroclimatic conditions, and yet responses akin to the Desert grasslands are possible in Plains grasslands with more extreme climate change. Collectively, these observations suggest that grasslands will undergo changes in function, feedbacks and sensitivity during prolonged warm, dry conditions characteristic of predicted climate change in grassland regions, and will serve as a bellwether of climate change.